Question

Which of the following enzymes facilitates the transport of citrate into bacterial cells?

• A. Citrate dehydrogenase
• B. Citrate permease
• C. Hexokinase
• D. Citrate phosphatase

Hint:

In flavor-producing starter cultures, citrate permease transports external citrate into the cell, where it is converted into flavor compounds like diacetyl.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Citrate metabolism by lactic acid bacteria, such as Leuconostoc and certain Lactococcus species, is important for producing flavor compounds (like diacetyl and acetoin) and carbon dioxide in fermented dairy products.
Citrate must first be transported across the bacterial cell membrane to enter these metabolic pathways.

Step 2: Detailed Explanation:
The translocation of citrate from the milk serum into the bacterial cytoplasm is facilitated by a specific membrane transport protein:
Citrate permease: This enzyme is a membrane-bound transport protein that catalyzes the active transport of citrate into the cell.
Once inside the cell, citrate is cleaved by citrate lyase into oxaloacetate and acetate.
Oxaloacetate is then decarboxylated to pyruvate, which serves as a precursor for diacetyl, acetoin, and 2,3-butanediol.
Other enzymes listed are not involved in this transport pathway:
Citrate dehydrogenase and citrate phosphatase do not exist as standard transport proteins for citrate.
Hexokinase is an intracellular glycolytic enzyme that phosphorylates hexose sugars and has no role in citrate transport or metabolism.

Step 3: Final Answer:
Therefore, citrate permease is the enzyme that facilitates the transport of citrate into bacterial cells.